Usually a number has no correlation to position on the pitch. A right back is a defender that plays on the right side. If you know what his favorite number he uses when he plays that in my opinion is better than just using number 2.
Unless you are completely sure what his favorite number is i would recommend leaving the jersey blank or choosing his favorite Fiorentina player/best most popular players name on the back. Example ( Rossi , Cuadrado, Gomez, Aquilani)
2) If he loves football, a jersey will be an amazing present. no doubt about that.
3) depends if he likes jersey slim fitting wear medium because it fits snug and tight if not i would say large to be good. I am 5'8 on the athletic side and i like a snug fit so i wear a medium usually.
Guy above me got it pretty much right and printing is quite expensive so make sure you know what his favorite number is and player for that team but yeah